The amplification of optical signals is of especial importance in optical communications. The radiation used in optical communications is not necessarily in the visible region, and the words "optical" and "light" when used in this specification are not to be interpreted as implying any such limitation. Indeed, if silica optical fibres are used as the transmission medium, infra-red radiation is of especial usefulness because the loss minima occur in such fibres at 1.3 .mu.m and 1.55 .mu.m approximately.
A semiconductor laser comprises in general an "active layer" of low band gap material with higher band gap "confinement" layers to either side and a p-n junction in the vicinity of the active layer. When current is passed from p- to n-type material, electrons and holes combine in the active layer to generate light. The threshold current at which the onset of "lasting" occurs depends on the degree of feedback into the structure, e.g. by reflections from its end faces, on the (current dependent) gain through stimulated emission as photons pass along the active layer, and on other factors. At lower currents, the laser functions as a light emitting diode or as a superluminescent emitter. A laser in which the feedback is provided by reflections at end facets is known as a Fabry-Perot laser.
It is known that a semiconductor laser structure can be used as an optical amplifier. Thus, it is known that if a laser structure is chosen having a wavelength of maximum gain close to the wavelength of the optical signal to be amplified and this signal is coupled into its active layer then it can be amplified if a driving current of less than the threshold value is passed through the structure from p- to n-type material. The phrase "laser structure" is used here and elsewhere in this specification to indicate the similarity of the amplifier structure to that of a laser without necessarily implying that lasing actually occurs in use.
We have given attention to the characteristic of such amplifiers that the amplifier gain for a given driving current is substantially constant from zero input power over a range of lower powers and then decreases to zero and then becomes negative (i.e. net absorption occurs) with higher powers. This is at least partly because the device saturates as the proportion of the available holes and electrons consumed increases. Increasing the driving current can be used to increase the supply of holes and electrons and therefore the gain; however, the extent to which this can be done is limited, because around the lasing threshold of the device injection-locked lasing occurs whereafter the dependence on the power input of the power output at the single emission wavelength is slight, i.e. useful amplification does not occur.
An object of the present invention is to provide means for amplifying higher input powers.
The present invention in its first aspect is based on our surprising discovery that the effects of saturation are less marked on the long-wavelength side of the wavelength of maximum gain as measured for lower powers.
In its first aspect the present invention provides a method of amplifying an optical signal which comprises coupling the optical signal to be amplified into the active layer of a semiconductor laser structure through which a driving current is passed, the amplified signal being emitted from the active layer, in which method the wavelength of the gain maximum at that driving current for the low power limit of optical power input .lambda..sub.max, the longer of the two wavelengths of zero gain at the driving current for the low power limit of optical power input .lambda..sub.upper, and the wavelength of the optical signal to be amplified .lambda. are related by the equation EQU .lambda..sub.upper &gt;.lambda.&gt;.lambda..sub.max.
In its first aspect the present invention further provides an amplification assembly for amplifying an optical signal which comprises an optical signal source, a semiconductor laser structure, means for coupling the optical signal to be amplified into the active laser of the semiconductor laser structure, and means for passing a driving current through the semiconductor laser structure, the amplified signal being emitted from the active layer in use, wherein the wavelength of the gain maximum of the semiconductor laser structure at that driving current for the low power limit of optical power input .lambda..sub.max, the longer of the two wavelengths of zero gain at the driving current .lambda..sub.upper, and the wavelength of the optical signal to be amplified .lambda. are related by the equation EQU .lambda..sub.upper &gt;.lambda.&gt;.lambda..sub.max.
Preferably, (.lambda.-.lambda..sub.max)/(.lambda..sub.upper -.lambda..sub.max) is at least 0.1, especially at least 0.2.
In a second aspect, our invention is based on our appreciation that if the input wavelength is sufficiently remote from the wavelength of he laser gain maximum to avoid injection-locking, whether on the high side or the low side, useful amplification of the input signal can be obtained with currents in excess of the threshold current.
In its second aspect, therefore, the present invention provides a method of amplifying an optical signal which comprises coupling the signal to be amplified into the active layer of a semiconductor laser structure and applying a driving current to the laser structure such that lasing occurs, the wavelength of the signal to be amplified being such that injection locking is avoided and the amplified signal is emitted from the active layer.
In its second aspect, the present invention further provides an amplification assembly for amplifying an optical signal which comprises an optical signal source, a semiconductor laser structure, means for coupling the optical signal to be amplified into the active layer of the semiconductor laser structure, and means for passing a driving current through the semiconductor laser structure such that lasing occurs, the wavelength of the signal to be amplified being such that injection locking is avoided and the amplified signal being emitted from the active layer.
In a third aspect, the present invention is based on our appreciation that the output of a laser structure under use as an optical amplifier is diminished over its entire emission range (not merely at the wavelength of the input signal) by the saturation referred to above and that this diminution offers a means of controlling the current so as to raise up the amplifier gain at higher input powers.
In this third aspect, therefore, the present invention provides a method of amplifying an optical signal which comprises coupling the signal to be amplified into the active layer of a semiconductor laser structure through which a driving current is passed, which layer emits the amplified signal, and monitoring the light emission from the active layer at a wavelength or over a wavelength range distinct from that of the amplified signal, and controlling the driving current to the laser structure so as at least partly to compensate for the decrease in the monitored output with increasing optical input.
In its third aspect, the present invention further provides an amplification assembly for amplifying an optical signal which comprises an optical signal source, a semiconductor laser structure, means for coupling the optical signal to be amplified into the active layer of the semiconductor laser structure, and means for passing a driving current through the semiconductor laser structure, the amplified signal being emitted from the active layer in use, wherein means is provided to monitor the output of the semiconductor laser structure at a wavelength or over a wavelength range distinct from that of the amplified signal, and a feedback control loop is provided which acts to control the driving current so as at least partly to compensate for the decrease in the monitored output with increased optical input power.
The invention can be employed independently in each of its three aspects, or in all three simultaneously, or in any of the three possible combinations of two aspects. In its first and third aspects, the invention can even be applied to a laser structure with so little feedback that it cannot be made to lase at any practical current (e.g. a travelling wave amplifier).